This invention relates to gas discharge laser systems and in particular to such systems having a seed beam for wavelength selection.
The natural bandwidth of many gas discharge lasers is relatively broad and many applications of such lasers require very narrow bandwidths. For example, state of the art integrated circuit lithography systems utilize KrF excimer lasers for which the bandwidth has been reduced from its natural width of about 300 pm to less than 1.0 pm.
FIG. 1 shows a cross section of the laser chamber of a KrF excimer laser currently used as an integrated circuit lithography light source. FIGS. 2A and 2B are combination block diagram and schematic views of a portion of the laser system. A laser gas (in this case a mixture of about 1.0% krypton, 0.1% fluorine and the rest neon) at 3 atmospheres is contained in chamber 10 and circulated with fan 50 past heat exchanger 11 and between two elongated electrodes 18 and 20 at speeds of about 2 cm per millisecond. A gain medium is created between the electrodes by high voltage discharges (about 16,000 to 20,000 volts) applied by a peaking capacitor bank of twenty-eight 0.59 nf capacitors 19 at a pulse rate of, for example, 1000 Hz. The electrodes and the peaking capacitor bank are the final stages of high voltage pulse power system which provides the high voltage pulse discharge energy from standard plant power source such as standard 230 volt AC source. This type of laser is described in detail in several patents including U.S. Pat. No. 4,959,840 which is incorporated herein by reference. High voltage power supplies for these lasers utilize magnetic compression circuits which produce the high voltage pulses with durations of about 50 ns. One such circuit is described in U.S. Pat. No. 5,936,988 which is incorporated herein by reference. The high voltage is transferred to cathode 18 through 15 brass high voltage feed throughs 21 from high voltage bus 23 to cathode support bar 53. Anode 20 is mounted on anode support bar 44 which is grounded utilizing ground return cage 52 comprised of rods about which do not substantially affect gas flow. In this embodiment, laser gas flow is directed with flow control vanes 46 and 47 and flow restrictor 49 constructed of Al2O3 insulator material.
In this prior art laser system, preionization is provided by two preionizers 56. The line narrowing of many of these state-of-the-art KrF excimer lasers is accomplished using a line narrowing package (LNP) 15 which consists of a three prism beam expander 7, a tuning mirror 14 and a grating 16 arranged in a Littrow configuration as shown in FIG. 2A.
This basic laser system configuration, over the past ten years, has been extremely successful so that these lasers are the light source for most new high production rate integrated circuit lithography machines. Several hundred are now in operation producing millions of IC chips annually. The laser systems are extremely reliable, typically operating around the clock with forced downtimes much less than 1%.
With the 248 nm wavelength and a bandwidth of about 0.8 pm from the KrF laser, lithography devices can produce integrated circuits with features as small as about xc2xc micron. However, there is a strong desire in the integrated circuit industry for further decreases in the feature sizes. A popular industry rule of thumb, known as Moore""s Law (Gordon Moore, former Chairman of Intel Corporation) is that the feature size is reduced by a factor of two every three years. Smaller bandwidths is one of several parameters which permit further reduction in feature size.
Several well-known techniques for reducing the bandwidth of gas discharge laser systems (including excimer laser systems) utilize two separate lasers. In one such system, the first laser called a xe2x80x9cmaster oscillatorxe2x80x9d is designed to provide a very narrow band beam and that beam is used as a seed beam in the second laser. If the second laser functions as a power amplifier, the system is referred to as a master oscillator, power amplifier (MOPA) system or a master-slave laser system. A primary advantage of the MOPA design as compared to the system shown in FIG. 2A is a substantial reduction of heat load on the LNP optics. If the second laser itself has a resonance cavity, the system is referred to as an injection seeded oscillator (ISO) and the seed laser is called the master oscillator and the downstream laser is called the power oscillator.
Laser systems comprised of two separate lasers tend to be substantially more expensive, larger and more complicated than comparable single laser systems. Therefore, commercial applications of two laser systems has been limited.
Systems have been proposed for using a single laser chamber to contain two sets of electrodes. For example, FIG. 3A shows a side-by-side arrangement described by Letardi in U.S. Pat. No. 5,070,513. Another arrangement shown in FIG. 3B described by Long in U.S. Pat. No. 4,534,035 in which the elongated electrode sets are positioned on opposite sides of the chamber. Gas flows from a common xe2x80x9cinxe2x80x9d plentum separately between the two sets of electrodes into a common xe2x80x9coutxe2x80x9d plenum. An arrangement proposed by McKee in U.S. Pat. No. 4,417,342 is shown in FIG. 3C. This system has two elongated electrode sets mounted parallel to each other on one half of the chamber. A tangential fan and heat exchanger is located in the other half. Gas flows in parallel through between the two sets of electrodes. The system shown in FIG. 3A has not been considered suitable for high pulse rate laser because debris from the upstream discharge interferes with the downstream discharge. According to an article published in Applied Physics B Lasers and Optics 1998, this laser is operated at a pulse repetition rate of about 100 pulses per second. The authors indicate that an attempt to operate at 1000 Hz would lead to turbulent flow which is not desirable for generation of a high quality beam. The system shown in FIG. 3C has not been considered suitable for high pulse rate lasers because splitting of the flow reduces the velocity of the gas between the electrodes by about 50% as compared to a single set of electrodes on the system shown in FIG. 3A. The system shown in FIG. 3B has not been considered satisfactory for high pulse rate lasers because the blower circulation is axial rather than tangential as shown in FIG. 1.
A need exists for a high pulse rate laser system combining concepts of the commercially successful single chamber, line narrowed gas discharge laser described above with the advantages of the MOPA on the ISO.
The present invention provides a single chamber gas discharge laser system having a pulse power source for producing electrical discharges at the rate of at least 1000 pulses per second. The discharge along with laser optics create two short lived gain media, one for producing a seed beam and the other for amplifying the seed beam. Laser gas circulation around a chamber circulation path is provided and the electrodes and discharges are arranged so that debris from one of the gain media is not circulated to the other gain media during discharges until the debris has made a loop around at least 90% of the chamber circulation path.